The present invention relates to a method and structure for forming an electrode on a light emitting device. More particularly, the present invention relates to the method and structure for providing a plurality of ohmic contact dots formed on a light emitting device.
In recent years, a great deal of attention has been directed to light-emitting devices utilizing gallium nitride-based III-V group semiconductors such as GaN, AlGaN, InGaN, and AlInGaN. Furthermore, a transparent sapphire substrate is usually used for such devices. Different from a conductive substrate used for the other semiconductor light-emitting device, sapphire is electrically insulated. Thus, it is not possible to mount, directly on the substrate, electrodes for supplying a predetermined current to the compound semiconductor layer causing the device to emit light. Both p-electrode and n-electrode must be formed in direct contact with the p-type compound semiconductor layer and the n-type compound semiconductor layer, respectively.
Referring to FIG. 1, a top view shows the conventional gallium nitride-based III-V group semiconductor light emitting device. Referring to FIG. 2, a cross-sectional view is taken along the line IVxe2x80x94IV of FIG. 1. The light-emitting device has a structure in which a layer of an n-type GaN 20, a layer of an n-type AlGaN 30, an active layer 40 (which is selected by using InGaN, AlInGaN or GaN to form the double hetero-junction or quantum well structure), a layer of an p-type AlGaN 50, and a layer of an p-type GaN 60 are all stacked on a sapphire substrate 10.
After etching process, a portion of the n-type GaN 20 is exposed. Then, the first electrode 70 and the second electrode 80 are formed respectively on the exposed n-type GaN surface 20 and on the exposed p-type GaN surface 60. The first electrode 70 comprises a metallic material. The metallic material that achieves preferable ohmic characteristics contains two metals of titanium formed in direct contact with the n-type GaN layer 20, and a layer of aluminum formed on the titanium layer. In order to obtain a perfect ohmic contact, annealing the metallic material layer is required. The annealing treatment is preferably conducted at a temperature of 400 degree. C. or more.
Because the carrier concentration of the p-type GaN is only 5xc3x971017/cm3, the second electrode 80, which is not similar to the small area of the first electrode 70, will cover the most part of the p-type GaN 60 exposed surface to spread the current. The second electrode 80 is formed to directly cover an entire exposed surface of the p-type GaN layer 60 for increasing the efficiency of the current spreading. But the second electrode 80 will shade the light emitting from the light emitting device. In this way, a thin second electrode 80 is formed on the p-type GaN 60 to transmit the light emitting from the light emitting device. A light transmitting electrode provided in contact with the p-type semiconductor layer is described in the U.S. Pat. No. 5,563,422. That is a gallium nitride-based III-V compound semiconductor device and method of producing the same. The second electrode 80 may be formed by any suitable metallic material. A particularly preferable metallic material contains gold and nickel. Gold and nickel are preferably formed such that a layer of nickel is formed in direct contact with the p-type GaN layer 60, and a layer of gold is formed upon the nickel layer. The annealing treatment is preferably conducted at a temperature of 400 degree. C. or more. A metallic material used for the second electrode 80 is preferably formed such that the annealed material has a thickness of 10 angstrom to 1000 angstrom. By adjusting the thickness of the second electrode 80 in the range of 10 angstrom to 1000 angstrom, the second electrode 80 can be rendered light-transmission. Due to the thin second electrode 80, a bonding pad 90 is contacted to the p-type GaN layer 60. The process of forming the bonding pad 90 is to firstly form a window 95 upon the second electrode 80 exposing the p-type GaN layer 60 surface. The bonding pad 90 is then formed covering portions of the second electrode 80 and adhering on the p-type GaN layer 60 surface.
Because the second electrode 80 is formed by metallic material, the process of forming the thickness of second electrode 80 should be seriously concerned. If the thickness of the second electrode 80 is thicker than that of expectation, most of the light emitting from the light emitting device will be absorbed by the second electrode 80 causing a poor transparent efficiency. If the thickness is thinner than that of expectation, it is difficult to have a second electrode 80 with good ohmic characteristics. Furthermore, the second electrode 80 of predetermined thickness formed on the p-type GaN layer 60, it is inevitable that a constant portion of the light emitting from the light emitting device will be absorbed by the second electrode 80 causing a low transparent efficiency of about between 60% and 80%.
Referring FIG. 3, a schematic diagram shows the conventional GaAs-based, InP-based, GaP-based, SiC-based or ZnSe-based light emitting device. The light emitting device includes at least an n-type substrate 96, an n-type semiconductor layer 98, an active layer 100, a p-type semiconductor layer 102, an n-electrode 104, and a p-electrode 106. Generally speaking, after the n-electrode 104 and the p-electrode 106 are formed, the annealing treatment is then processed. Consequently, regions of high light absorption are formed on the ohmic contact area, and cause the difficulty of fabricating a device with higher output efficiency.
It is therefore an object of this invention to provide a method and structure for forming an electrode on a light emitting device. The present invention provides a brand-new method and structure to form the transparent electrode or reflective electrode on a p-type gallium nitride-based compound semiconductor. The electrode comprises a plurality of opaque ohmic contact dots formed on the p-type gallium nitride-based compound semiconductor and a transparent conductive layer (or a light reflective conductive layer) covering the p-type gallium nitride-based compound semiconductor.
It is another object of this invention to provide a method and structure for forming an electrode on a light emitting device. Comparing with the conventional electrode formed on p-type GaN-based III-V compound semiconductor, the present invention has advantages of higher light penetration and easier in process. Moreover, utilizing the present invention, the electrode is suitable for any light emitting devices. The output efficiency of the light emitting device is higher than that of a conventional light emitting device. Furthermore, this process of forming the electrode is easier than that of the conventional process.
In accordance with all aspects of this invention, this invention provides a structure for forming an electrode on a light emitting device, comprising: a semiconductor layer of a light emitting device having a first surface and a second surface, a plurality of ohmic contact dots formed on said first surface, and a conductive layer covering said ohmic contact dots and said first surface.
In accordance with the aforementioned objects of this invention, this invention provides a method for forming an electrode on a light emitting device, comprising: forming a plurality of contact dots on the surface of a semiconductor layer of a light emitting light device, carrying out an annealing treatment, forming a conductive layer covering said contact dots and said surface.